This invention relates to a permanent magnet, direct current machine so constructed that the frame will accomodate a larger and more powerful armature relative to the interior and exterior dimensions of the frame. This description is equally applicable to permanent magnet, direct current motors and generators. For purposes of illustration, the particular disclosure of this application is to motors.
Permanent magnet DC motors have a very wide range of application because of the relatively large amounts of power which can be produced within a relatively small, lightweight structure. This type of motor is constructed within a frame which forms the motor housing. The frame may be circular, square or other cross-sectional shapes depending on the number of poles. Permanent magnets are attached around the interior walls of the frame, leaving sufficient space for the armature to be mounted therein for rotation in close, spaced-apart relation to the magnets. In circular housing motors, the magnets are generally arcuate so that they mate against the inside of the housing. In square frames the magnets are flat on one side and are glued against the planar side walls of the frame, or may be triangular and mounted in the corners of the frame.
The motor armature typically has a plurality of windings which are connected to an external power source through brushes and a mechanical commutator, or through one of many types of brushless commutators. The commutator causes the voltage to be applied selectively to the armature winding so that the magnetic field produced in the armature on the average will be at a 90.degree. angle to the stator magnetic field produced by the permanent magnets. Because the angle between the armature field and the stator field in a DC motor is on the average 90.degree., torque on the armature is theoretically at its maximum.
Since the stator magnetic field in permanent magnet motors is generated by permanent magnets, no power is used in the field structure. The stator magnetic flux therefore remains essentially constant at all levels of armature current resulting in a linear speed-torque curve over an extended range. For many applications this is a substantial advantage over a comparable wound-field motor. With particular regard to a shunt motor, armature reaction flux tends to follow the low-reluctance path through the pole shoe. Higher current levels cause an angular shift in pole position and a lower effective flux level. Since permanent magnets of the type used in permanent magnet motors have an extremely high coercive force, the magnet material will resist any change in flux whenever the armature reaction field enters. An additional advantage in permanent magnet motors is that since electrical power is not supplied to generate the stator magnetic flux, power requirements are lower since the conversion of electrical power to mechanical power in windings always results in a heat loss in the winding itself. The permanent magnet motor therefore inherently simplifies power supply requirements and, at the same time, requires less cooling. In the invention disclosed herein, these design characteristics of permanent magnet motors are further enhanced.
Because of the ability of DC motors to generate relatively large amounts of torque at operating speed, they are in ever greater demand for applications such as robotics where the motor must create a relatively large amount of torque within a small space.
Heretofore the combined thickness of the permanent magnet and the frame has been a substantial limiting factor in the construction of smaller and more powerful permanent magnet DC motors. Electric motor design is based upon the placement of conductors in a magnetic field. Each turn of a conductor wound into a coil adds to the magnetic field intensity which exists in the space enclosed by the coil. Therefore, the power of a motor depends largely upon how much magnetism, or magnetic flux, there is in the space around the permanent magnet or in the air gap of a motor. Magnetic flux in turn depends on several other factors, including the magnetic flux density, permeability and magnetic field intensity present in the motor. Magnetic flux density is a measure of concentration of magnetic flux in an area. Permeability is the degree to which a medium will support a magnetic field. Relative permeability is used to describe the ability of different materials to support magnetic fields. Air has a relative permeability of one, while ferromagnetic materials such as iron or steel have a relative permeability of several hundred. Most motors are therefore built with iron or steel since a magnetic circuit which contains ferrous materials may have several hundred times the flux of one constructed with non-ferrous materials. Finally, magnetic field intensity describes how magnetomotive force is used around a magnetic circuit.
All of these factors intimately affect the construction of the motor frame. Even if materials of a sufficient strength can be found, the frame cannot be made too thin because the mass of the frame and the magnetic permeability of the frame materials must be sufficient to properly transmit the flux generated by the permanent magnet and by the armature.
The power produced by a motor is proportional to the diameter of the armature, squared, times the length of the armature. A motor can be lengthened while maintaining the same cross-sectional size to increase its power. In many cases, however, the length of the motor cannot be increased because of size limitations. Heretofore the only other practical way of increasing the power of the motor without increasing the length of the armature has been to increase the cross-sectional dimensions of the motor in order to accommodate a larger diameter armature. Again, a more powerful motor is produced but only in a larger size. Therefore, this application relates to the selective decrease in thickness of the motor frame to form recessed seats within which the permanent magnets are placed. Because of the placement of the magnet seats, the transmission and flow of flux through the motor at various points is altered, resulting in greater motor efficiency.